The present invention relates to a fuel cell and a system thereof. More specifically, the invention pertains to a reversible type polymer electrolyte fuel cell capable of treating a regenerative current and a system thereof.
FIG. 2 is a schematic view illustrating the constitution of a conventional polymer electrolyte fuel cell.
As an ion exchange membrane which is a main component of the polymer electrolyte fuel cell, a cation exchange membrane called a “proton exchange membrane” 6 is usually employed. Catalyst layers 4,5 are disposed on both sides of this proton exchange membrane 6. As these catalyst layers 4,5, a paste having a platinum or platinum alloy catalyst borne on carbon particles is often used. A gas diffusion layer 2 is disposed on the outside of each catalyst layer. This diffusion layer 2 is usually made of carbon paper and it is also called collector layer through which electrons generated on the catalysts are allowed to pass.
On the outside of the gas diffusion layer, there exists a plate having a gas channel 3 therein, and it is called “interconnector” 1. The interconnector 1 is made of a current-carrying substance and has the function of separating a fuel gas from an oxidizing gas. The fuel gas is usually composed of hydrogen gas and water vapor. The catalyst layer brought into contact with this hydrogen-gas-containing fuel gas is called “hydrogen electrode”. The term “oxidizing gas” as used herein means an oxygen containing gas such as air and the catalyst layer brought into contact with an oxidizing gas is called “oxygen electrode”.
When this solid electrolyte fuel cell is connected with an external load, a hydrogen gas (H2) is decomposed into protons (H+) and electrons (e−) at the hydrogen electrode 4 and the protons transfer through an electrolyte membrane toward the oxygen electrode 5 side. After the transfer, the protons combine with oxygen (O2) and electrons, which have flown into the oxygen electrode 5 side, to produce water (H2O) on this side. Fuel cells circulate electrons (e−), which have been exchanged during water formation reaction processes through an external circuit, thus acting as a battery.
At this time, protons (H+), together with water molecules therearound, transfer in the form of clusters. This is so called electroendosmosis of water. With this phenomenon, the membrane on the hydrogen electrode 4 side dries up. For the transfer of protons, existence of water is indispensable. On the dried film surface, ion resistance increases and disturbs the passage of ions. In order to overcome this problem, the hydrogen gas at the hydrogen electrode 4 is usually fed with externally humidification (H2+H2O), which suppresses an increase in electric resistance caused by the drying of the membrane. Water carrying capacity of the hydrogen gas depends on the water vapor pressure. The higher the gas temperature, the greater the water vapor pressure and a larger amount of water can be fed into the fuel cell.
An excessive rise of water vapor pressure, on the contrary, decreases the partial pressure of hydrogen (H2) itself, thereby exerting a bad influence on the performance of the fuel cell. The amount of water to be supplied from the outside externally therefore has a certain limit.
At the oxygen electrode 5, an amount of water becomes excessive owing to electroendosmosis of water and generation of water. This creates a water concentration gradient between the hydrogen electrode 4 side and oxygen electrode 5 side of the membrane, leading to a phenomenon called “back diffusion” which is physical diffusion of water toward the hydrogen electrode 4. A portion of this excessive water is exhausted outside by evaporation as a gas at the oxygen electrode 5.
If more water exists, however, it cannot be controlled by evaporation or diffusion and overflows as condensed water through the diffusion layer 2 or gas passage 3 of the interconnector 1. This phenomenon is called “flooding”, which prevents oxygen gas from reaching the membrane 6, thereby deteriorating the performance of the fuel cell.
Some countermeasures have been studied in consideration of the above-described problems, but they are not completely free from the problems.
Ordinarily employed external humidification involves the above-described problem. Physical or mechanical circulation of water within stacks has also some problems. As other countermeasures, (1) the promotion of back diffusion of water by making the membrane thinner, thereby increasing the magnitude of water concentration gradient, (2) the generation of water in the membrane, and (3) the humidification within the passage by infiltrating cooling water into the interconnector are known.
Countermeasure (1) is however accompanied with a problem of physical permeation of a raw gas called “cross flow of gas”. Such a reaction arises aside from the main reaction of a fuel cell and prevents part of electrons from flowing into an external circuit. This lowers the electromotive force of the cell. In countermeasure (2), the humidity of the membrane is adjusted by dispersing the catalyst in the membrane and converting the crossflow gas into water. In this method, however, pinholes appear in the film owing to the reaction heat and deteriorate the life of the fuel cell. Countermeasure (3) involves the same problem as the one caused by the external humidification and in addition, some difficulty in gas sealing may occur.